The present invention relates to a device for supporting the rachis, installed by surgery. Said device can be used just for consolidating a rachis which needs to be consolidated (accidental fracture, for example) or else for correcting and supporting a rachis suffering from curvatures (scoliosis, kyphosis, for example). Although the device according to the invention is suitable for these two types of applications as well as others, it is described hereinafter primarily with reference to the surgical treatment of lateral curvatures of the spine.
Scoliosis, it is known, is a lateral curvature of the spine, together with rotation of certain vertebra about their axis. Abnormal spinal flexure, resulting from said lateral curvature, is generally defined from specific points, namely the upper and lower extreme vertebra between which said abnormal flexure is found, and the vertebra or vertebra at the top thereof. Said upper and lower extreme vertebra are those which have pivoted the least about their axes, but they are on the other hand, those most inclined on the median longitudinal axis, and their planes determine between them the angle of scoliotic curvature. The apical vertebra or vertebra are those farthest from said median axis of the torso; they are slightly inclined or not inclined at all with respect to said axis but, on the other hand, they are those which suffer the strongest rotation about their axis.
When the scoliotic curvature angle exceeds a certain threshold (around 50.degree.), it becomes necessary to consider treating the scoliosis by surgery, said surgical treatment being called arthrodesis and consisting in welding together all the vertebra of the scoliotic curvature, after a maximum correction thereof, by straightening and opening.
Such correction may be accomplished before surgery by continuous traction of the rachis or by using corrective plasters. Said correction however, is completed and finalized during surgery. To this effect, a solid prop member is placed in the concavity of the rachidial curvature at least, said prop member consolidating the resulting correction and being able to strengthen the bone joint obtained by arthrodesis.
One instrumentation called the "HARRINGTON instrumentation" is already known to perform this. Said instrumentation consists of an elongation system, designed to be inserted in the concavity of the curvature, and of a compression system designed to be optionally inserted on the convex side. Often in fact, only the elongation system is used.
Said elongation system comprises two metallic anchoring members, of the hook type, resting against the extreme vertebra of the curvature, and a metallic pin acting as a support, and serving to keep the hooks one apart from the other, and as a result, to correct the scoliotic curvature. Said metallic pin, of circular cross-section, is provided at its lower part with a shoulder member, and at its upper part, with a notched portion, and it traverses the said hooks. The lower hook is fastened on the lamina of the lower extreme vertebra of the curvature and the metallic pin rests on said lower hook via the said shoulder member. The upper hook, which is fitted over the said pin, is placed under the facet of the upper extreme vertebra of the curvature and by external operations of traction (using for example retractors which rest aginst the notches of the upper pat of the rod), the upper hook is drawn apart from the lower hook. Said upper hook goes past the notches of the notched portion of the pin, successively, and when the required correction is obtained, a locking device (such as a clip for example) is inserted between the upper hook and the notch nearest thereto.
The compression system is composed of transverse hooks traversed by a threaded pin provided with nuts. The upper hooks are fastened on the transverse processes of the vertebra, whereas the lower hooks rest under the transverse processes or under the lamina of the vertebra of the curvature which are close to the lower extreme vertebra. A nut is associated with each hook, and the required compression is obtained by actuating said nuts.
After fitting the instrumentation into place, an arthrodesis is carried out and the patient is made to wear a corset for support for a period varying between 6 and 12 months.
The type of instrumentation, although widely used, presents a number of disadvantages, which are that:
(a) the opening of the curvature cannot be adjusted in continuous manner, but on the contrary in step-wise manner, each adjusting step being constituted by a notch of the notched portion of the elongation system. As a result, final adjustment of the opening of the rachis is achieved, not absolutely accurately, but only to within the nearest notch;
(b) opening of the curvature is achieved by applying very localized pressure at the level of the two extreme vertebra of the curvature only, so that the pressure exerted on said vertebra is very strong;
(c) the hooks of the elongation system, in particular, can pivot about the metallic pin of said system, thus enabling them to move with respect to their initial positioning;
(d) no direct re-centering action towards the axis of the torso is exerted by the instrumentation;
(e) no real action of derotation is exerted on the apical vertebra or vertebrae;
(f) despite prolonged postsurgical support, breakage of the rod of the elongation system occurs, at the junction of the smooth parts with the notched parts, in 2.5 percent of cases.